1. Field of the Invention
The present invention relates to the rechargeable battery arts and more particularly to nickel zinc rechargeable battery design.
2. Description of the Related Art
The advent of portable communication and computation devices that allow diverse mobile connectivity has fueled growth and innovation in the rechargeable battery field. Increased capacity and power has made possible the entry of rechargeable power sources in various fields including the power tool arena. Because power tools typically have large current demands, rechargeable power sources have necessarily evolved to accommodate rapid discharge characteristics. It can be appreciated that the present invention will also find use in applications other than power tools such as Uninterruptible Power Supplies (UPS), Electric Vehicles, and high demand consumer electronics—all of which require high carrying capacity and current discharging ability. Of course, the invention also applies to relatively lower discharge rate applications such as many mainstream consumer electronics applications.
Because of the hazardous nature of some of the commonly used materials for conventional rechargeable power sources, it would be desirable to manufacture a rechargeable power source that reduces the quantity of any potentially hazardous materials. In particular, it would be desirable to find a substitute for the widely used nickel cadmium (NiCd) battery cell.
The rechargeable nickel zinc (NiZn) cells provide a power-to-weight ratio comparable to and even exceeding nickel cadmium cells at a reasonable cost. However, nickel zinc battery technology has not been widely deployed in part because it has been found to have a relatively limited cycle life. In other words, a given nickel zinc cell can only charge and discharge for a fraction of the cycles typically attained with a comparable nickel cadmium cell. This is due to zinc distribution and dendrite formation. Various advances in electrolyte composition and other chemistry improvements have reduced these issues, but they remain an important consideration in cell design.
The negative thermodynamic potential of zinc relative to cadmium and the metal hydride electrode, while allowing NiZn batteries to provide more power density than NiCd and Ni-metal hydride batteries, presents additional issues that are not addressed by conventional battery designs. First, nickel zinc batteries are more likely to evolve hydrogen. In addition, the main conductive paths associated with the negative electrode in conventional NiCd batteries are nickel plated steel. Nickel plated steel is not compatible with the negative zinc electrode.
There is a need therefore for a nickel zinc battery design that takes advantage of the improved performance offered by the battery chemistry, yet has a long cycle life and that is able to prevent hydrogen build-up.